Why Marsupials 'Mate Themselves to Death': Better Sperm

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The males of several species of insect-eating marsupials have a
very unusual life history. They mature quickly, often in less
than a year. Then, during a short and frenzied breeding season,
they mate repeatedly — for up to 14 hours in some cases — until
their immune systems crash and their bodies start to fall apart.
Then they die. (Talk about going out with a bang.)

This strategy, called suicidal reproduction, or semelparity, is
seen in some animals and plants, but is quite rare in mammals,
said Diana Fisher, a researcher at the University of Queensland,
Australia. The behavior is only found in these marsupials, a
group of
mammals in which young can be carried in a pouch.

Most animals that pursue this tactic, like squid and spiders,
have many offspring, making it more understandable from an
evolutionary point of view: Though they die after mating, they
likely have thousands offspring to which they pass on their
genes, Fisher told LiveScience. But marsupials, like most
mammals, only have a few offspring at a time. [ Top
10 Swingers of the Animal Kingdom ]

Not altruistic

When the behavior was first noted in the mid-20th century amongst
marsupials in
Australia and several Pacific Islands like New Guinea, scientists
were perplexed. At first, some thought the behavior could be a
type of altruism; by dying, the males left more insects to be
eaten by their offspring, Fisher said.

But recent research, culminating in a paper published today (Oct.
7) in the journal Proceedings of the National Academy of
Sciences, shows that males live fast and die young for another
reason: to fertilize females with the highest quality sperm, in
order to pass on their genes to as many offspring as possible.

In the study, Fisher and colleagues compared suicidally
reproducing (or semelparous) marsupials with other marsupials
that mate on multiple occasions throughout life (iteroparous).
The researchers found that the semelparous animals have much
larger testes relative to body size, which are better for storing
sperm, Fisher said. In fact, these semelparous males actually
stop producing sperm before mating begins, so they can put all of
their energy into marathon mating sessions. For instance, male
Antechinus, a group of
mouselike marsupials, can mate for 14 hours and even break
down their muscles to give them more energy, Fisher said.

"They must rely on stored sperm throughout the mating season, and
the clock is ticking, because the limited store of sperm is
continually lost in their urine," Fisher said. "The have a
frenzied mating season lasting only a couple of weeks, and males
usually die before young are born."

Live fast, die young

Scientists also found that semelparous species have shorter
breeding seasons, so males have a short time to "sow their wild
oats," so to speak.

All of this leaves males exhausted, and they usually die of
infections or internal bleeding — they "mate themselves to
death," Fisher said. The mating is timed so that offspring are
born just before the largest number of insects emerges, in the
summertime, she added.

Instead of duking it out like males of some species, like
bighorn
sheep, for access to females, the males compete with their
sperm. Females usually mate with multiple males and may sire
offspring from multiple fathers. But the best sperm wins — and to
maximize the quality, the males sacrifice everything, Fisher
said. Previous work has shown that promiscuous
female Antechinus, which mate with multiple males,
have more offspring than those with a single partner.

"This is a form of post-mating sexual selection in which males
compete with their sperm inside the female reproductive tract,
rather than fighting to gain access to females," Fisher said. "It
is associated with having large testes with lots of
sperm-producing tissue, and often also long mating times and
mate-guarding, and of course promiscuity."

The sperm-competition idea also explains why males mate for so
long — to prevent, or at least minimize, the amount of time other
males can mate with their partner, Fisher added.